70 FRAGMENTS OF SCIENCE 



the simplest character, is still easy to grasp. When air is 

 permitted to rush into an exhausted tube, the temperature 

 of the air is raised to a degree equivalent to the vis viva 

 extinguished. 1 Such air is said to be dynamically heated, 

 and, if pure, it shows itself incompetent to radiate, even 

 when a rock-salt window is provided for the passage of its 

 rays. But if instead of being empty the tube contain a 

 small quantity of vapor, the warmed air communicates its 

 heat by contact to the vapor, the molecules of which con- 

 vert into the radiant form the heat imparted to them by 

 the atoms of the air. By this process also, which I have 

 called Dynamic Radiation, the reciprocity of radiation and 

 absorption has been conclusively proved. a 



In the excellent researches of Leslie, De la Provostaye 

 and Desains, and Balfour Stewart, the same reciprocity, as 

 regards solid bodies, has been variously illustrated; while 

 the labors, theoretical and experimental, of Kirchhoff have 

 given this subject a wonderful expansion, and enriched 

 it by applications of the highest kind. To their results 

 are now to be added the foregoing, whereby gases and 

 vapors, which have been hitherto thought inaccessible to 

 experiments with the thermo-electric pile, are proved by 

 it to exhibit the indissoluble duality of radiation and 

 absorption, the influence of chemical combination on 

 both being exhibited in the most decisive and extraor- 

 dinary way. 



1 See page 19 for a definition of vis viva. 



5 When heated air imparts its motion to another gas or vapor, the trans- 

 ference of heat is accompanied by a change of vibrating period^ The 

 Dynamic Radiation of vapors ia rendered possible by this transmutation of 

 vibrations. 



